When a hydrogel is placed in a solvent it absorbs solvent until it reaches an equilibrium swelling when the reactive forces balance the swelling forces. Any binding process that modifies the affinity of the polymer chains to the solvent will change the equilibrium swelling. Alternatively, any interaction that modifies the degree of cross-linking in the polymer network will also change the equilibrium swelling. The binding processes may also modify the refractive index of the hydrogel, either through a changed volume fraction of polymer or through the presence of additional molecules bound to the polymer chains.
This change in volume and/or refractive index which such hydrogels exhibit when a particular analyte is present can be measured to determine the concentration of that analyte in the solvent. Various methods can be used to measure these changes in the hydrogel but optical measurement and particularly interferometric measurement is an especially attractive technique since it allows dimensional changes to be accurately measured. The hydrogel forms a Fabry-Perot optical cavity from which an interference signal is produced.
Various optical sensor devices using hydrogel swelling have been proposed. For example, U.S. Pat. No. 5,898,004 (Polymerized Crystalline Colloidal Array Sensor, Asher et. al.) disclose a device composed of crystalline colloidal array polymerized in a hydrogel. The hydrogel swelling is measured as a shift in the Bragg diffraction wavelength. U.S. Pat. No. 5,611,998 (Optochemical Sensor And Method For Presentation, Aussenegg at. al.) discloses a sensor composed of small metallic islands on top of a thin layer of swellable polymer backed by a mirror. The film structure is characterized by strong narrow reflection minima whose spectral position is sensitive to the polymer thickness. Various sensor devices are also reported in the art. For example, Seitz et. al. (“Derivatized, swellable polymer microspheres for chemical transduction,” Analytica Chimica Acta, vol. 400, pp 55-64, 1999) disclose a device composed of swellable polymer microspheres incorporated in a hydrogel. Turbidity spectra are affected by the microsphere swelling.
Various fiber optical chemical and physical sensor devices using thin polymer layers at the optical fiber end face have also been reported. These prior approaches have many limitations. Sensors based on polymer films, or drops, are too dense to allow macromolecules to diffuse in to the sensor material. Furthermore, sensors based on fluorescence, scattering or intensity modulation are made to avoid specular reflection from the polymer surface since specular reflection will interfere with the sensor signal. On the other hand previously proposed interferometric sensors all use polymer sensor material in the form of thin planar layer or disk on the fiber end. The inventors have appreciated that these have a less than optimal optical coupling to the fiber waveguide mode; and a less than optimal speed of diffusion response. It is an object of the invention to improve these characteristics.
Our earlier PCT application published under number WO03/034047 discloses various arrangements for the interferometric measurement of swelling and/or refractive index changes of a hydrogel mass at the end of an optical fiber. Also disclosed in that application is a technique for making such fiber sensors which comprises covering a pre-gel material with a lid before gelation.